PS: The point here is NOT that poor people shouldn't have kids, it's that no one should be having kids without having the basics of childcare covered, which necessitates widespread systemic, societal, and economic change so that no parent needs to worry about those basics, and kids can grow up in secure, healthy environments. I completely forgot to mention this when I originally posted it, and that's my bad.
One of my kids is a genetics scientist and she explained this as - it's not that older women have a higher rate of conceiving sick kids, but our immune systems do a worse job of figuring that out and aborting them. So 22 year old gets pregnant, embryo is not healthy, body yeets it before she even knows she's pregnant. 42 year old body doesn't screen as well and it might stick.
Anecdotally - I had one set young & one in late 30s and there is no difference in their health or intelligence. The one difference I think the older ones got was watching me go to school and get a better job, they were around for that economic arc and sort of internalized the belief it can be done. Well, and the second set were bigger because I was, as the midwife put it, better nourished as we were not so poor.
And I cannot with confidence say that my kids harmed my economic situation as they were the reason I had to get a better job. Probably wouldn't have bothered otherwise.